Method and apparatus for detecting the presence of dangerous concentrations of combustible gases or vapors in the air

ABSTRACT

Method and apparatus for the continuous detection of relative amounts of combustible gases and vapors in ambient air and wherein the ambient air flows through a reaction chamber in contact with an oxidation catalyst in the presence of a plurality of detecting means sensitive to the varying radiant energy emitted along said catalyst in the direction of flow of the ambient air and means for receiving and comparing these different detected values.

0 United States Patent 11113,553,461

[72] Inventors Matteo Siano [56] References Cited Y q l l UNITED STATESPATENTS 6 'g f; tf 2,878,388 3/1959 Bergson 350/206X 2]] A l N 1 2 Y3,312,826 4/1967 Finkle 250/218 f z- N 3 1967 2,879,663 3/1959 Thomas73/26X 45] P f t d 3,238,519 3/1966 Ramsey.... 340/237 a e 3,087,7954/1963 Ross 340/237x Primary Examiner-William F. Lindquiist [54] METHODAND APPARATUS FOR DETECTING Leedm THE PRESENCE OF DANGEROUSAttorney-Bowyer and wltherspoon CONCENTRATIONS 0F COMBUSTIBLE GASES gg gIN m I ABSTRACT: Method and apparatus for the continuous detecalms rawmgtion of relative amounts of combustible gases and vapors in [52] U.S.Cl. 250/218, ambient air and wherein the ambient air flows through areac- 23/232,23/254, 23/255, 250/218,:340/237 tion chamber in contactwith an oxidation catalyst in the [51] lnt.Cl ..G0ln 21/26 presence of aplurality of detecting means sensitive to the [50] Field of Search250/218; varying radiant energy emitted along said catalyst in the73/26; 340/237(lnquirecl); 23/232E, 254E, direction of flow of theambient air and means for receiving 255E(Inquired) and comparing thesedifferent detected values.

DIFFERENT/4L IMPL lF/EE I PATENTEU JAN 5 I971 LEEE y; Q .T

METHOD AND APPARATUS FOR DETECTING THE PRESENCE OF DANGEROUSCONCENTRATIONS OF COMBUSTIBLE GASES OR VAPORS IN THE AIR It is knownthat, when the concentration of a combustible gas or .vapor in theambient airattains a certain minimum value, called the lower limit-ofinflammability" or LLl, a danger of spontaneous ignition of theair-combustible mixture exists; in like manner when the concentration ofa toxic gas or vapor in the ambient air attains a certain maximum value,viz, the maximum tolerable limit (MTL) there exists a danger ofintoxication. The ignition limits for the hydrocarbons are known, aswell as MTL, and this particularly is true for the aromatichydrocarbons.

This invention has as its object a method and apparatus of detecting inthe ambient air the presence of a gas or vapor which is combustible,such as a hydrocarbon, or toxic, such as carbon monoxide, by theemission of a signal when the concentration of the gas or vapor reachesa certain safety value.

This value is lower than the lower inflammability limit (or LLI) of thegas or vapor on the basis of a safety coefficient, which is determinedin advance.

In the ambient air and in presence of a gas concentration (or vaporconcentration) which is equal or greater than the lower limit ofinflammability (LLI) there exists a danger of ignition, from any heatsource whatsoever (lighted cigarette, strongly heated metallic parts,and so on) when it brings the temperature of the mixture to thespontaneous ignition temperature (SIT) of the fuel. This willimmediately promote the ignition of the mixture and the propagation ofthe flame with consequent swift increase of the pressure and explosion.

On the other hand, there is no ignition danger when the concentration ofthe gas or vapor is lower than the LL], because, whatever the heatsource may be, no ignition reaction will be possible, due to the lack ofthe necessary kindling energy for obtaining the reaction, in view of theinsufficient fuel mass.

The problem of the emission of a signal when the concentration of thegas or vapor attains a certain safety value is therefore equivalent tosolving the problem of rendering possible the continuous observation ofthe variations of the activation energy of the oxidation reactions ofthe mixture, prior to its ignition.

These reactions do not generate any light and only a small amount ofheat. Generally it might be said that, up-to-date, there is no apparatuswhich performs a direct and continuous observation.

The Wheatstone-bridge devices, for example, indicate the electricaleffect (resistance variation) within the catalyst filament due to theheat generated by the reaction; but this effect does not measure thisamount and the indications are indirect and rough.

Other apparatus, which are usually influenced by the rela tive humidityof the ambient air, present considerable inconveniences and limitations.

This invention aims to overcome the drawbacks of the known devices bymeans of a method and apparatus permitting the continuous observation ofthe variations of the activation energy of the oxidation reactions ofcombustible gases or vapors, within the concentration limits of safetywithin the ambient air, independently of the variations of the humidityand in a very accurate manner.

In practice, the oxidation reactions take place within a catalyticreaction chamber having an almost stationary thermodynamic behavior andthe stationary energy status (relative variation of activation energy)which corresponds to the safety concentration, is signaled byphotosensitive devices which are suitably arranged and connected to thereaction tube.

The peculiar characteristic features and advantages of the inventionwill be apparent from the following specification of an embodiment ofthe invention, described with reference to the accompanying drawing,wherein:

FIG. I shows diagrammatically an apparatus according to the invention;and

FIG. 2 shows, in cross section, a detail of the reaction chamber.

Referring particularly to the drawing, 1 denotes the reaction tubewhich, in the example as shown, is made of glass. One end of the tube 1is connected to a feed pump P for the air under examination, while theopposite end opens to the atmosphere. The tube 1 is enclosed in asuitable envelope 2 made of good heat-conductive material, for examplealuminum, and is provided with fins 3 for heat dissipation.

In the envelope 2 two small chambers or pockets 4 and 5 open towards thebore 6 of said tube 1. In each of said pockets 4 and 5 a photosensitivecell, 7, 107, is mounted. Coaxially within tube 1 a thin platinumhelical filament 8 is mounted. The helical filament 8 has such a lengthas to extend in front of the photosensitive elements 7 and 107. B is anelectric battery for feeding current to the filament 8..

The photosensitive elements 7, 107 are connected to a suitabledifferential amplifier AD, the output of which is utilized forcontrolling a relay R which, upon being sufficiently energized, closesthe usually open contact 9 of an alarm circuit comprising, for example,a battery BI and the bell 10.

To the output of the amplifier AD some recording, measuring, indicatingor other apparatus M :may be connected.

It is to be noted that within the tube 1, which contains a heat source,a fuel-air mixture flowing therethrough under the action of the heatforms a thermodynamic reaction system which is almost stationary.Experience has shown that the partial concentrations and pressures ofthe system are not equal in the different points within the tube 1. Thisis in contrast with an approximately insulated system in which to eachtemperature value there corresponds an almost uniform distribution ofenergy (thermodynamic equilibrium). Within tube 1 there is no suchuniform energy or temperature distribution.

It is still to be noted that from an electrical view point, and asexperience has shown, the above-described tube is intrinsically safe, asin it no ignition of combustible mixture can take place, whatever theconcentration of'the fuel may be.

It is thus possible to effect along the metallic catalyst and upon avery thin skin thereof taken as support, different oxidation reactionsfrom one spot to another, as the respective concentrations andtemperatures are different. It is possible to detect these AIR ofradiating energy generated by said reactions by means of photosensitivecells 7 and 107.

Said phenomena take place as follows:

UNPOLLUTED AIR I The unpolluted air, i.e. the air which does not containa combustible gas or vapor, is filtered! before being sucked by the pumpP and is blown continuously from this pump (which may be a membrane pumphaving an adjustable output) through tube 1 at a slightlysuperatmospherical pressure. The metallic catalyst 8, being energized byits own energy source B is rendered incandescent or, in other words, iscaused to emit, due to the activation of the valence electrons of themetal luminous radiations within the visible spectrum with a wave lengthA of 0.8 to 0.4 p. (micron). 'l:l 1ese radiations, through energy quantaE h- 7 h a quantity of energy (heat) which increases with the frequencyv and is reduced with the increase of the wave length of said radiationsand the heat quantity thus set free (metal incandescence status) areproportional to the energy furnished to the metal.

As the amount of this energy is small, the luminous emission, during thenormal operation of the apparatus, is in the band of the red. But Adepends upon the composition of the atmosphere (c is light propagationvelocity in the vacuum); whereas the ambient air is a mixture ofnitrogen (about 78 percent by volume), oxygen (about 20 percent) and acertain amount of water vapor and consequently, within the tube 1 thereis an interaction between the radiations (photons) emitted by the metaland the corpuscles of the ambient air (electrons loosely bound to theatoms), which has for its effect to reduce the frequency (and toincrease the wave length) of the radiations.

The radiations (generally within the short infrared band) strikingagainst 7 and 107 are the result of this interaction and represent thestatus of the energy taken as starting level of the observation.

POLLUTED AIR (ambient air containing some combustible gas or vapor).

The gas or vapor and the air react along the catalyst 8 and the valuesof fuel concentration, temperature of the mixture and reaction heat aredifferent from place to place. Namely, they are greater in front of 7than in front of 107 and the different energy values (with respect toone another and to the starting level) in front of 7 and 107 depend uponthe fact of the different transitions of the atoms (their valenceelectrons) during the respective reactions, viz. they depend upon thedifferent frequency of the emitted radiations.

By the photosensitive devices 7 and 107 the radiations striking againstthem are transduced to electric values, magnified by AD and, upon therelative energy attaining a certain (adjustment) value which correspondsto the safety concentration of the polluting gas or vapor, relay Rcloses the switch G and the alarm bell rings.

The catalyst may be made either of a transition metal, such as platinum,palladium, nickel or cobalt, having a compact structure and a largeactive surface area and in form of helical or coiled filament, or by twoor more of said components, forming an alloy, or by one of them actingas catalyst and another substance (for example, thorine) acting askindler, whose function is to enhance the catalytic action, even if thekindler is, per se, inactive; or also be a mixture of substancesproducing together an effect of synergism. The catalyst, with or withoutkindler or synergistic effect may be made in a variety of manners, forexample, upon a support which may be either metallic (such asnickel-chrome) or of suitable ceramic material (such asaluminosilicates).

The photosensitive elements 7 and 107 may be constituted by photovoltaiccells made of semiconductors (silicium, germanium and so on), saidsemiconductors being chosen so as to give a suitable spectral reactionin a determined band, or else by other devices for the directtransformation of luminous energy into electric energy.

Finally it may be noted that a rougher detection may be effected bymeans of thermocouples welded to the catalyst filamentor by thermistorssuspended within the tube.

We claim:

1. A method for continuously detecting the relative amounts ofcombustible gases or vapors in the ambient air comprising the steps of:

1. continuously flowing ambient air through a longitudinal chamber incontact with a longitudinally disposed oxidation catalyst;

2. continuously heating the oxidation catalyst to produce continuous andsuccessive oxidation reactions along the longitudinally disposedcatalyst whereby radiant energy is emitted from said reactions inresponse to the combustible gases and vapors in the ambient air at everypoint along the catalyst;

. measuring the emitted radiant energy at a plurality of longitudinallydisposed points along the path of the ambient air in the longitudinalchamber; and i 4. determining the differences between measurements atthe aforesaid points and applying said differences to prescribedstandards to determine the relative amounts of combustible gases orvapors in the ambient air.

2. Apparatus for the continuous detection of the relative amounts ofcombustible gases and vapors in the ambient air said apparatuscomprising a reaction vessel, said vessel having a longitudinallydisposed reaction chamber, an oxidation catalyst longitudinally disposedin said chamber, means for activating the catalyst, means for flowingambient air through the longitudinally disposed reaction chamber incontact with the oxidation catalyst thereby causing the catalyst to emitradiant energy in response to the combustible gases and vapors in theambient air, two radiant energy detecting units ositioned inlongitudinal spaced relation adjacent the catalyst 0 sense radiantenergy emitted from the catalyst, and means to receive detected valuesfrom the detecting units and compare the differences against aprescribed standard.

3. The invention as set forth in claim 2 and wherein an alarm isprovided in the apparatus and is actuated when the detected differencesexceed prescribed values.

4. The invention as set forth in claim 3 and wherein the reaction vesselis provided with a pair of longitudinally spaced pockets, each pockethousing one of the radiant energy detecting units. I

5. The invention as set forth in claim 2 and wherein the means toreceive the detected values and compare the differences against aprescribed standard includes a differential amplifier.

6. The invention as set forth in claim 2 and wherein the oxidationcatalyst is a platinum wire.

7. The invention as set forth in claim 2 and wherein the catalyst iscoated on a suitable support member.

8. The invention as set forth in the reaction vessel is provided withcooling fins.

2. continuously heating the oxidation catalyst to produce continuous andsuccessive oxidation reactions along the longitudinally disposedcatalyst whereby radiant energy is emitted from said reactions inresponse to the combustible gases and vapors in the ambient air at everypoint along the catalyst;
 2. Apparatus for the continuous detection ofthe relative amounts of combustible gases and vapors in the ambient airsaid apparatus comprising a reaction vessel, said vessel having alongitudinally disposed reaction chamber, an oxidation catalystlongitudinally disposed in said chamber, means for activating thecatalyst, means for flowing ambient air through the longitudinallydisposed reaction chamber in contact with the oxidation catalyst therebycausing the catalyst to emit radiant energy in response to thecombustible gases and vapors in the ambient air, two radiant energydetecting units positioned in longitudinal spaced relation adjacent thecatalyst to sense radiant energy emitted from the catalyst, and means toreceive detected values from the detecting units and compare thedifferences against a prescribed standard.
 3. The invention as set forthin claim 2 and wherein an alarm is provided in the apparatus and isactuated when the detected differences exceed prescribed values. 3.measuring the emitted radiant energy at a plurality of longitudinallydisposed points along the path of the ambient air in the longitudinalchamber; and
 4. determining the differences between measurements at theaforesaid points and applying said differences to prescribed standardsto determine the relative amounts of combustible gases or vapors in theambient air.
 4. The invention as set forth in claim 3 and wherein thereaction vessel is provided with a pair of longitudinally spacedpockets, each pocket housing one of the radiant energy detecting units.5. The invention as set forth in claim 2 and wherein the means toreceive the detected values and compare the differences against aprescribed standard includes a differential amplifier.
 6. The inventionas set forth in claim 2 and wherein the oxidation catalyst is a platinumwire.
 7. The invention as set forth in claim 2 and wherein the catalystis coated on a suitable support member.
 8. The invention as set forth inthe reaction vessel is provided with cooling fins.